Conventionally, as one form of crime prevention device, MW sensors are known (Japanese Laid-Open Patent Publication No. Hei 7-37176 for example) in which microwaves are transmitted toward a detection area and, when a figure is present in the detection area, the figure (intruder) is detected by receiving the waves (microwaves modulated by the Doppler effect) reflected from that figure.
Moreover, as one type of MW sensor, sensors are known in which a distance to an object is measured using a plurality of microwaves of different frequencies. This type of sensor is set up so that, for example, two microwaves of different frequencies are transmitted toward a detection area, and the phase difference is detected between two IF signals based on the reflected waves of each of these. This phase difference correlates to the distance to a target (a detection target object such as a human figure), with a tendency for the phase difference to be greater for longer distances to the target. In other words, it is possible to measure the distance to the target by obtaining this phase difference. The following is a description concerning an operation for detecting the phase difference in IF signals in this type of sensor.
When the IF signals, which are based on the reflected waves of two microwaves of different frequencies, are sine waves IFout1 and IFout2 (with a phase difference corresponding to the distance to the target) as shown in FIG. 3(a), the rectangular waves A and B formed from these IF signals are as shown in FIG. 3(b). It is then possible to measure the distance to the target by detecting the phase difference (the phase difference Δt of the rising flank portion of the rectangular waves in the drawing) between the rectangular waves A and B.
Incidentally, when using this type of sensor as a crime prevention sensor, it is preferable that only human figures (intruders) are detected, and that other objects such as small animals are not detected. FIG. 2 shows an example of a relationship between the distance from the sensor when a detected object is a human figure and when the detected object is a small animal (such as a cat), and the signal level of the reflected waves.
As evident in this drawing, when the detection object is a human figure, the signal level is lower when the figure is at positions farther from the sensor. On the other hand, in the case of the detection object being a small animal, when the animal is present at a position an extremely short distance from the sensor, the signal level of the reflected waves is low. This is caused by the directivity of the receiver antenna of the sensor. That is, when a small animal is present at a position an extremely short distance from the sensor, the signal level is low since the receiver antenna receives the reflected waves diagonally from below where directivity is poor. In the case of a small animal being present at a position that is to a certain extent a far distance from the sensor, then, as in the above-described case of a human figure, the signal level is lower at positions farther from the sensor. However, since small animals are of a small size compared to human figures, the signal level of their reflected waves is lower as a whole.
Incidentally, with conventional MW sensors, an object detection signal is transmitted (reported) when a signal level above a preset trigger level is input.
For this reason, it is necessary to set a low trigger level when attempting to also be able to detect intruders that are present at comparatively far distances (a position of 8 m, for example). However, in this case, the probability becomes higher that small animals will be detected. For example, with a trigger level that is set as shown by the broken line in FIG. 2, although intruders that are present at comparatively far distances are detectable, small animals that are present at a position of about 3 m from the sensor also are detected.
Conversely, when setting the trigger level high so as to not detect small animals, intruders that are present at comparatively far distances cannot be detected. For example, with a trigger level that is set as shown by the dot-dash line in FIG. 2, although small animals are not detected, the ability to detect intruders that are present in the vicinity of 7 m from the sensor is lost.
The present invention has been devised giving consideration to this issue, and it is an object therein to provide a MW sensor that detects objects using microwaves with the ability to discriminate the size of a detection object and to offer a MW sensor that can judge whether or not the detection object is of a size that makes it a detection target, and is thus able to avoid false reports.